gcc: Build MinGW stage two with threading library

Currently this is set up to be mcfgthreads, but it could be something
else instead.

pkgs/development/compilers/gcc/common/configure-flags.nix

@@ -62,7 +62,7 @@ let
       "--enable-__cxa_atexit"
       "--enable-long-long"
       "--enable-threads=${if targetPlatform.isUnix then "posix"
-                          else if targetPlatform.isWindows then "win32"
+                          else if targetPlatform.isWindows then "mcf"
                           else "single"}"
       "--enable-nls"
       "--disable-decimal-float" # No final libdecnumber (it may work only in 386)

pkgs/development/compilers/gcc/common/extra-target-flags.nix

@@ -1,4 +1,4 @@
-{ stdenv, crossStageStatic, libcCross }:
+{ stdenv, crossStageStatic, libcCross, threadsCross }:
 
 let
   inherit (stdenv) lib hostPlatform targetPlatform;
@@ -12,6 +12,7 @@ in
         "-B${lib.getLib dep}${dep.libdir or "/lib"}"
       ]);
     in mkFlags libcCross
+    ++ lib.optionals (!crossStageStatic) (mkFlags threadsCross)
     ;
 
   EXTRA_TARGET_LDFLAGS = let
@@ -24,5 +25,6 @@ in
           "-Wl,-rpath-link,${lib.getLib dep}${dep.libdir or "/lib"}"
       ]));
     in mkFlags libcCross
+    ++ lib.optionals (!crossStageStatic) (mkFlags threadsCross)
     ;
 }

pkgs/development/compilers/gcc/common/mfcgthreads-patches-repo.nix

@@ -0,0 +1 @@
+"740f233da00c4fb5bcc225b2e29768824bcecc58"